The title acid, 3-amino-2-methylpropanoic acid monohydrate, C4H9NO2.H2O, crystallized in the centrosymmetric space group Pbca in the zwitterionic form. The three H atoms on N, which are involved in hydrogen bonding, are ordered. The three intermolecular N-HO hydrogen bonds have NO distances ranging from 2.758 (2) to 2.809 (2) Å and N-HO angles ranging from 149 (2) to 171 (1)°. The two intermolecular O- HO hydrogen bonds have OO distances 2.739 (2) and 2.755 (2) Å and O- HO angles 170 (2) and 175 (2)°. Each acid molecule and its associated water molecule are directly hydrogen-bonded to five acid molecules and two water molecules; the structure comprises two subsets of molecules which are not cross-linked by these hydrogen bonds. Through basic second-level graphs, approximately two-thirds of the hydrogen-bonding patterns are finite and one-third are chains; there is a single ring pattern, which occurs about a center of symmetry.